Combined implantable ventricular defibrillator and pacemaker stimulation devices are well known in the art. Such devices permit a heart to be paced for treating bradycardia, for example, while also detecting for ventricular fibrillation and ventricular tachycardia and applying defibrillating electrical energy, cardioversion shocks or antitachycardia pacing pulses to the heart when fibrillation or tachycardia is detected.
One problem that must be addressed in such devices is the need to reliably sense R waves. To this end, implantable cardiac devices generally include an automatic sensing control. The aim of such control is to maintain the sensing threshold low enough (sensitive enough) for detecting low amplitude R wave electrical activity of the heart (as may be present during fibrillation) while avoiding over-sensing which could result in a T wave or noise being sensed by the device and mistaken for an R wave.
Automatic sensing control has been performed by first establishing a ventricular refractory period (VREF) upon sensing an R wave and continuing the VREF for a pre-determined time such as 100 to 140 milliseconds. Following the VREF, the sensing threshold is set at an initial level and then decreased thereafter from the initial threshold level to a minimum threshold level where it is held until the next paced or sensed event. The initial threshold, refractory period, threshold decay rate, and minimum threshold are selected so that the threshold is above the amplitude of the T waves or noise when they occur.
Even though present sensing controls have provided improved performance in rejecting far field signals and T waves in the sensing of R waves problems of inappropriate sensing and detection still exist. This can result in delivery of inappropriate therapies and significant patient morbidity.
Unfortunately, existing sensing controls still require programming to fix initial threshold, refractory period and threshold decay rate values. Unfortunately, these methods do not account for drifting of T wave amplitude and location. T wave amplitude and location can vary from patient to patient or with the same patent as the status of the patient's disease changes or even with changes in heart rate. This can cause a T wave to be present with an amplitude which exceeds a sensing threshold and thus be mistakenly detected as an R wave.
Thus, what is needed is an automated sensing control which takes T wave characteristics into account when controlling the sensing threshold.